Estimates of HVAC filtration efficiency for fine and ultrafine particles of outdoor origin

Epidemiology studies have consistently shown associations between increased adverse health effects and elevated outdoor fine particulate matter mass (PM_2.5) (Brook et al., 2010, Miller et al., 2007, Pope and Dockery, 2006, Pope et al., 2002) and ultrafine particle number concentrations (UFPs: particles <100 nm in size) (Penttinen et al., 2001, Stölzel et al., 2007, Von Klot et al., 2002, Weichenthal et al., 2007). However, the majority of exposure to fine and ultrafine particles of outdoor origin often occurs inside buildings (Allen et al., 2004, Bhangar et al., 2011, Kearney et al., 2011, Meng et al., 2009, Meng et al., 2005). This is because outdoor particles can penetrate indoors (Chen and Zhao, 2011) and people in industrialized countries spend much of their time indoors (Jenkins et al., 1992, Klepeis et al., 2001). Indoor particle control technologies such as stand-alone air cleaners and particle filters installed in central heating, ventilating, and air-conditioning (HVAC) systems are being increasingly relied upon to reduce indoor concentrations of particles of both indoor and outdoor origin (Brown et al., 2014, Howard-Reed et al., 2003, MacIntosh et al., 2010, MacIntosh et al., 2008, Offermann et al., 1985, Riley et al., 2002, Stephens and Siegel, 2012a, Stephens and Siegel, 2013, Wallace et al., 2013, Wallace et al., 2004, Zaatari et al., 2014).

Although much of the health concerns with outdoor particulate matter are associated with PM_2.5 and, to a lesser extent, UFPs, common HVAC filter test standards do not explicitly account for either of these measures when evaluating particle removal efficiency (ASHRAE, 2012, CEN, 2012). Rather, they tend to characterize removal efficiency on a size-resolved basis, as is appropriate for fibrous filter media because filtration efficiency varies widely by particle size (Hinds, 1999a). For example, the most widely used particle filtration test standard in the U.S., ASHRAE Standard 52.2, classifies the single-pass particle removal efficiency of HVAC filters based on the minimum removal efficiency for three particle size bins (0.3–1, 1–3, and 3–10 μm) under various loading conditions in a laboratory test facility (ASHRAE, 2012). Minimum removal efficiency values in these three size bins are then used to assign HVAC filters a single efficiency metric called the Minimum Efficiency Reporting Value (MERV). The assignment of the MERV metric to minimum particle removal efficiencies for the three size bins in Standard 52.2 is provided in Table S1 in the SI.

Three shortcomings are apparent in ASHRAE Standard 52.2 and its resulting MERV metric. First, although particle size bins 1 and 2 evaluate removal efficiency for particle sizes within the PM_2.5 size range (0.3–1.0 and 1.0–3.0 μm, respectively), there is no explicit reference to PM_2.5 mass concentration removal efficiency. PM_2.5 mass removal efficiency will vary highly depending on particle size distribution (PSD) and particle density (El Orch et al., 2014, Hanley et al., 1994, Riley et al., 2002). Although one recent study found strong correlations between E₁ removal efficiency and indoor and outdoor origin PM_2.5 mass removal efficiency (Zaatari et al., 2014), it was limited by a small number of outdoor PSDs and PM_2.5 mass concentrations. A larger number of outdoor PSDs is important to capture because they can vary widely by location (e.g., rural, urban, and close to traffic), season (e.g., winter, spring, summer, and fall), or even time of day (e.g., morning, afternoon, and nights) (Costabile et al., 2009, Jaenicke, 1993, Kelly et al., 2011, Puustinen et al., 2007, Seinfeld and Pandis, 2006, Virtanen et al., 2006).

Second, ASHRAE Standard 52.2 does not evaluate UFP removal, although there is some evidence to suggest that UFP removal efficiency tends to increase with MERV (El Orch et al., 2014, Hecker and Hofacre, 2008, Stephens and Siegel, 2012b). UFPs are important to capture because the vast majority of outdoor particles actually exist in the UFP size range (Hinds, 1999b, Seinfeld and Pandis, 2006). Finally, because of the lack of removal efficiency requirements for particle size bins 1 and 2 for many MERV assignments (i.e., MERV 1-8 for E₂ and MERV 1-12 for E₁), two different filters with the same MERV can have vastly different efficiencies for particles smaller than 3 μm (and particularly so for particles smaller than 1 μm). As an example, Hecker and Hofacre (2008) reported removal efficiency of different MERV 12 filters measured in laboratory tests to range from as low as 10% to as high as 70% for ∼100 nm particles.

Given these issues, there remains a need to improve knowledge of how different MERV filters perform in removing PM_2.5 and UFPs of outdoor origin. Improvements in knowledge of how MERV relates to PM_2.5 and UFP efficiency for outdoor particles can simplify indoor air quality modeling efforts and improve our ability to inform standards and guidelines. For example, there are currently draft proposals to increase filtration requirements in ASHRAE Standards 62.1 (ASHRAE, 2010) and 62.2 (ASHRAE, 2013) based on health outcomes of PM_2.5 (and possibly UFPs). Given these limitations, the objective of this paper is to provide estimates of particle removal efficiency of various HVAC filters for PM_2.5 and UFP of ambient origin. We achieved this by mapping 194 outdoor PSDs found in the literature to size-resolved particle removal efficiencies of a wide range of HVAC filters, including MERV 5, 6, 7, 8, 10, 12, 14, 16 and HEPA filters. We use the results to explore statistical distributions of outdoor-origin PM_2.5 and UFP removal efficiencies. We also test the sensitivity of our results to key assumptions such as particle density and modification by typical size-resolved infiltration factors in residences (in the absence of HVAC filtration), which alter outdoor PSDs as particles transport indoors.